A turbine engine for an aircraft generally comprises, from upstream to downstream in the direction of flow of the gases, a fan, one or more compressor stages, for example a low-pressure, LP, compressor, and a high-pressure, HP, compressor, a combustion chamber, one or more turbine stages, for example a high-pressure turbine and a low-pressure turbine, and a gas exhaust pipe. A turbine can correspond to each compressor, the two being connected by a shaft, thus forming, for example, a high-pressure, HP, body and a low-pressure, LP, body.
The shafts are supported upstream and downstream by bearings which are received in enclosures which shield them from the rest of the engine. The enclosures thus contain roller bearings which are interposed between a rotating member of the engine and a fixed part which supports the member, or else between two rotating parts, the two parts rotating at different rotational speeds, such as a pivot pin which is rigidly connected to the HP shaft and the LP shaft. The bearings are lubricated and cooled by oil. The oil, which is sprayed by the rotating parts, forms a mist of suspended droplets there. These enclosures are formed and delimited by walls of the fixed structure of the engine but also by the rotating elements. Sealing means are provided in the regions where the fixed and movable parts join together. The means must allow the passage of an air flow therethrough, for the purpose of pressurizing the enclosure and retaining as much oil as possible within the enclosure. This is why the sealing between the fixed and rotating elements of an oil enclosure is a particularly tricky problem.
The sealing is commonly achieved by using a labyrinth seal, which is the simplest, strongest and most widespread sealing solution for turbine engines. Such a seal comprises, firstly, knife-edge seals, or thin ribs, which are rigidly connected to a rotating part and, secondly, an abradable material, positioned opposite the knife-edge seals, which is rigidly connected to a fixed part. Since in this case a roller bearing is nearby, a clearance is imposed between the knife-edge seals and the abradable material so that the knife-edge seals do not hollow out the abradable material and do not create chips with the material which forms the abradable material: the roller bearings are sensitive to the metal particles which may damage them. Each knife-edge seal creates a loss of pressure in cooperation with the abradable material which faces the seal, and it is the sum of these pressure losses which ensures the required sealing. Other sealing techniques are also possible, such as brush seals, as described in the patent application in the name of the applicant, FR 1261694, in which a labyrinth seal is associated with a brush seal in order to control the rates of flow of leaks through the seal irrespective of the engine speed. The patent application FR 2 929 325 in the name of the applicant relates to a bearing enclosure, of which the rate of flow of leaks can be checked by checking the pressure inside the enclosure likewise according to the speed. In this application, seals of the segmented radial type are mentioned. The application of this type of seal to a turbine engine is described in the patent in the name of the applicant EP 387,122.
A bearing enclosure generally comprises two seals along the shaft, one upstream of the bearing which is contained in the enclosure, the other downstream of the bearings. However, some bearing enclosures can comprise one or more additional seals, and the enclosure itself can comprise a plurality of bearings. As indicated above, these seals are passed through by a gas flow from the outside of the seals towards the inside of the enclosure in order to prevent, during the operation of the engine, oil contained in the enclosure from escaping and polluting the other members of the engine. This gas is air originating from a source of pressurized air, in particular compressors.
The enclosure can be in communication with the open air and kept at a pressure which is close to atmospheric pressure. The bearings inside the enclosures are bathed in a mist of oil which is continuously extracted from the enclosure and separated in an oil separator.
The enclosure can also not be in direct communication with the open air and not comprise oil separation. A pump for recovering oil which is connected to a recovery port, which is located at 6 o'clock, at the low point of the engine, recovers the oil and the air from the enclosure and thus creates an intake of air through the seals of the oil enclosure. The pump advantageously has a pumping rate which is greater than the rate of flow of oil into the enclosure which allows the lubrication of the bearing. In this case, it is important to have an air flow through the two upstream and downstream seals, in order to retain the oil in the region of the two seals. And, so that there are air flows passing through the two seals of the oil enclosure, it is necessary to have a balance of pressure upstream of the two seals. By means of this balance of pressure, no preferred route is created which would favor one seal over the other and would thus compromise the sealing performance of the latter seal.
The present disclosure aims to solve the problem of balancing the pressures outside the two seals of the enclosure by increasing the pressure upstream of the seal, the pressure level of which is the lower of the two.
According to a known arrangement, the air which is dedicated to pressurizing the seals, which comes from the compressors, enters the chamber in which the bearing enclosure is located through an opening which is located close to the shaft, then is guided along the outer surface of the bearing enclosure radially then axially through suitable passages as far as the downstream chamber in order to supply the downstream seal. The analysis of the pressure levels of this pressurizing air flow shows that there is a pressure gradient between the air inlet opening and the region which is located at a higher radial level. This pressure gradient results from the recompression vortex in this chamber which is located upstream of the upstream seal of the bearing enclosure. The expression “recompression vortex” denotes the phenomenon which links the radial gap to a difference in pressure when a rotating flow is present. In this case, the flow becomes a rotating flow because it is driven by the rotation of the shaft of the turbine engine. As the pressurizing air flow coming from the compressor is introduced into the chamber through an opening which is located radially in the region of the shaft, the rotation of the shaft causes the rotation of this air flow which swirls radially as far as an annular discharge duct which is located at a greater radius than that of the opening for entry into the chamber. This swirling movement creates a radial pressure gradient on the ventilation air flow.